The present invention relates to a sodium-sulfur battery having a high reliability, which battery is suitable for various battery systems, such as those found in power storage equipment, electric vehicles, emergency power supplies, uninterruptible power supplies, a peak shift apparatus for electric power systems, frequency-voltage stabilizers, and other equipment, and to a battery system using the same.
A sodium-sulfur battery using sodium for the negative electrode and sulfur for the positive electrode active material has received widespread attention, because of its preferable efficiency and large energy density, and this type of battery is expected to be useful for power storage systems, electric vehicles, and other applications. However, the sodium-sulfur battery has a problem in that corrosive sulfur and sodium polysulfide cause corrosion of the cell container for the positive electrode, resulting in deterioration of the characteristics of the battery, which becomes a barrier to practical use of this battery. That is, the sodium-sulfur battery has a problem in that, when the surface of the metallic vessel forming the cell container for the positive electrode, which is made of a metal, such as stainless steel, is corroded by sulfur and sodium polysulfide, the sulfur, which is a positive electrode active material, is consumed in the formation of corrosion products, and the amount of the positive electrode active material necessary for the battery reaction is decreased, to thereby lower the battery capacitance. Further, there is another problem in that an effect of the electrical resistance of the metallic sulfide generated at the surface of the positive electrode increases the internal resistance of the battery, to thereby lower the efficiency of the battery. In order to solve these problems, various methods, wherein the inner wall of the cell container for the positive electrode is coated with a corrosion resistant coating agent composed mainly of Cr, Mo, Ti, Al, C and so on, have been disclosed. However, because of peeling due to thermal cycles and defects in the coating layer, the reliability of the coating is not sufficient in comparison with a case when a bulk material is used for the cell container for the positive electrode. For instance, JP-A-2-142065 (1990) discloses a cell container for the positive electrode made of an aluminum alloy, for instance, the surface of which is coated with a cobalt base alloy film containing 20.about.40 wt. % Cr, 1.about.3 wt. % C, and other materials by a plasma spraying method. The above case, wherein the surface of the cell container for the positive electrode was coated with a corrosion resistant Co base alloy film, had problems in that the adhesion and the durability of the coating film fluctuated readily because the manufacturing method is complex, and so the reliability was not sufficient because sometimes the coating film was peeled off during the assembling of the battery and during its operation. The coating alloy layer formed by the plasma spraying method readily absorbs gases generated from the molten metal, because the coating film is formed by solidification of molten metal. Thus, the alloy layer has a susceptibility to swelling or peeling off due to the partial pressure of the gases in response to a temperature rise during operation of the battery. Once the swelling or the peering occurs, the cell container for the positive electrode, which is made of an aluminum alloy, forms an insulating film by contacting the molten sodium polysulfide, and a problem occurs in that the efficiency of current collection from the cell container for the positive electrode decreases. Further, when an aluminum alloy is used for fabricating the cell container for the positive electrode, the carbon contained in the Co base alloy layer formed by the plasma spraying method reacts with the aluminum base material of the positive electrode with the heat produced during the plasma spraying to generate aluminum carbide (Al.sub.4 C.sub.3). The carbide reacts with water in the atmosphere to generate methane by a reaction shown by the following equation: EQU Al.sub.4 C.sub.3 +12H.sub.2 O=4Al(OH).sub.3 +3CH.sub.4
Therefore, handling of the cell container for the positive electrode in the atmosphere causes peeling off and deterioration of the alloy layer. Accordingly, consideration must be given to the fact that an operation for assembling the cell container for the positive electrode into the battery must be performed in an inert atmosphere, but this is deemed a disadvantage from the point of view of mass production of the battery.
Once Al.sub.4 C.sub.3 is generated, the alloy layer is susceptible to peeling off due to stress generated by the assembling or operation of the battery because of the brittleness of Al.sub.4 C.sub.3. As examples of using a bulk metallic material for the cell container for the positive electrode, various cases using Fe alloys containing a large amount of Cr have been disclosed, for instance, in JP-A-59-165378 (1984), JP-A-57-57861 (1982), and JP-A-56-130071 (1981). In preparing a cell container for the positive electrode using the above metallic material, welding is the most preferable method for finally sealing the cell container for the positive electrode from the point of view of reliability and operability. However, with the welding method of the prior art, the corrosion resistance of the material at the welding portion became worse than the bulk material. Accordingly, there was a problem in that the actual corrosion rate of the cell container for the positive electrode by sulfur and sodium polysulfide became faster than a value expected from a result of an experimental corrosion test. As a result, insufficient reliability of the batteries and reduction of the life expectancy because of a change in the corrosion resistant property depending on a variation in the welding conditions have remained as problems to be solved. Examples using welding for a Fe group cell container for the positive electrode were disclosed in JP-A-2-144858 (1990), JP-A-61-10881 (1986), and JP-A-48-43129 (1981). A corrosion resistant coating material was used for the cell container for the positive electrode disclosed in JP-A-2-144858 (1990). However, the corrosion resistant property of the material at the welding portion decreased significantly due to melting of the coating layer during the welding, and the low reliability of the battery was a problem which remained to be solved. On the other hand, JP-A-61-10881 (1986) disclosed a positive electrode lid made of stainless steel, Fe--Cr--Al alloy, or Fe coated with Al, and a positive electrode supplementary lid made of stainless steel, Fe--Cr--Al alloy, or Fe--Cr--Al--Y alloy. However, that disclosure did not suggest any material for the battery vessel, which was a key component of the cell container for the positive electrode, nor any content of Cr and C of the Fe alloy composing the positive electrode lid, which was another key component of the cell container for the positive electrode, and the positive electrode supplementary lid. In accordance with JP-A-48-43129 (1973), SUS 304 (Cr 18.about.20 wt. %, Ni 8.about.10.5 wt. %, Fe balance) was used as a material for the cell container for the positive electrode. Although SUS 304 has a preferable weldability, a sufficient reliability can not be obtained because of its poor corrosion resistant property against sulfur. The Fe alloy has a larger residual strain at the welding portion and a smaller corrosion resistance in comparison with a Co base alloy and Ni base alloy, and the specific resistivity of ferrous sulfide, which is a corrosion product of the Fe alloy, is higher than that of cobalt sulfide and nickel sulfide, and so the battery resistance readily increases . Therefore, in order to fabricate a reliable cell container for the positive electrode using a welding method, the composition of the Fe alloy used in the fabrication must be restricted exactly to a suitable range. However, prior approaches have not considered the above restriction exactly.